While you don't always have to use such a specialized lens, it helps when you want to keep the background soft, which brings attention to your subject, using a technique called selective focus or using a shallow Depth-of-Field.

If I am trying to execute selective focus (where the subject is sharp but the background is blurry) I take these steps:

Use the longest lens possible.

Use the widest aperture that your lens or the situation allows.

Try to put less distance between the camera and the subject.

Try to put more distance between the subject and the background.

This is not always something you can achieve when photographing wildlife but when you can, take the time move your position around to get those ratios in distance.

I also make sure my autofocus is on target. I used a product called Focus Tune to find tune my 300mm lens to make sure it works well with each camera body I own. This tedious process, gives me the confidence that when my autofocus hits on the eye of the animal I am photographing, I KNOW its going to be sharp.

Finding the Great Horned Owl is the real trick to this photograph. Once you learn of the species in your area, you need to exercise great skill in your approach to observe his daily routines, to get yourself into a position to capture great images of the animal.

In this particular case, I walked into the Sacramento Zoo and right near the entrance was this owl, resting on a handler's gloved hand. I did take the time to ask, when no one was visiting, to move around a bit, that way I could get the subject farther from the background for a better shallow Depth-of-Field shot. Also, moving around a few feet I could also get the owl to be in the best light.

The zoo is a great way to practice with wildlife that you know is there. While there are other challenges you will face in the zoo, enclosures, people setting their drinks on your camera bag, its a fabulous place to work on your wildlife photography skills.